1. Field of the Invention
This invention is directed to photo-detector structures, in general, and to a photo-detector array device including charge integrating photo-elements, in particular.
2. Prior Art
In typical photo-detector array architectures known in the art, the process of measuring the stored charge on a photo-element (typically, a reverse-biased PN junction) involves discharging the photo-element. In this instance, when the accumulated charge is small, the signal-to-noise ratio (SNR) of this measurement is poor. To reduce noise uncertainties to levels which permit dynamic ranges, i.e. maximum intensity versus minimum intensity, of more than 10.sup.4 to 10.sup.5 requires digital integration of many measurements and usually takes hours to accomplish.
A wide variety of photo-detector arrays exist on the market today. These include arrays of a number of individual, very closely spaced, photo-detectors fabricated on a single piece of silicon. This arrangement allows measurement of the intensity of a light beam as a function of displacement. In some cases, these arrays are used in acousto-optic spectrum analyzers wherein the displacement of the light beam along the linear dimension is proportional to the frequency of the light beam and the intensity is proportional to the signal strength at that frequency. Typically, this type of detector uses a charge-coupled device (CDD) detector array. In this arrangement, a number of reverse biased diodes, fabricated in a single chip, accumulate charge which has been generated by electron/hole pair generation when a photon strikes the material of the detector. The accumulated charge is loaded into a charge-coupled-device register and the charge packets, one for each diode, are transferred along the CCD to a charge sensing amplifier. (This type of device is often referred to as a "bucket brigade register".) The output voltage of the amplifier is proportional to the charge that has been dumped onto it. The charge is, in turn, proportional to the intensity of light which struck the photo-diode which produced the charge originally.
The principal drawback for many applications is that this type of detector is quite noisy. The noise is, typically, one part in ten thousand (or even as much as one part in a thousand) of the maximum signal, i.e. saturation level or intensity, that the detectors are capable of measuring. For many applications it is necessary to have less noise because the noise limits the smallest signal which can be reliably measured with the system. In a typical CCD detector array the noise is 10.sup.-3 of the maximum signal whereas in most applications it is desirable that noise be no more than 10.sup.-6 of the maximum signal. The smaller noise level allows measurement of intensities from a standardized unit of one to a million intensity units. One way that users of CCD detectors overcome this noise problem is to take many readings and then average these readings. This procedure reduces the uncertainties due to the noise so that measurements can be made accurately with very small intensities as well as with very large intensities. However, to take enough samples with a conventional CCD detector so as to reduce the noise by averaging takes a long time, e.g. on the order of an hour. This is unacceptably slow for many situations.
There is also known a non-integrating detector array which is capable of a dynamic range of about 10.sup.6. However, because it is a non-integrating detector, it can provide only an instantaneous reading of the intensity. This is a drawback in certain situations where both current and past information is required. That is, in general, a non-integrating detector forces the system to monitor all detectors simultaneously. Therefore, the multiplexing provided by a CCD becomes impossible. The ability to record past information accurately allows for considerable simplication of the system. In many cases the system could not be realized without such simplification.
In an application such as measurement or detection of frequency hop signals, it is frequently necessary to know what happened in the previous millisecond or ten milliseconds. However, a non-integrating photo-detector does not permit this type of operation.